Frame accurate splicing

ABSTRACT

A video delivery system alters information (such as time stamp information) associated with one or more frames of a first compressed video file to allow for accurate rendering of frames within the first compressed video file prior to a transition to a second compressed video file. For example, a time stamp of a particular frame of a compressed video file is altered prior to transmission of the particular frame by a video transmission system such that the altered time stamp indicates a time that has already passed. The particular frame can be decoded by a display system that receives the particular frame so that information included in the particular frame can be used in rendering of subsequently received frames. The display system can bypass display of the particular frame due to the altered time stamp.

TECHNICAL FIELD

This document generally relates to splicing compressed video into asingle presentation.

BACKGROUND

In the past, multimedia content presentations, such as televisionprograms, are formatted to include segments of primary media content(i.e., the television show) and empty segments in which secondarycontent such as station identifiers, news updates, location or regionspecific content, and other content can be inserted. Insertion wasperformed by changing transmitting the secondary content during anappropriate time relevant to the primary media content. As streamingdigital media has become more common, compression based file formats forstoring and transmitting video data have been developed.

SUMMARY

This document describes techniques, methods, systems, and othermechanisms for splicing two segments of compressed video data to providea single video presentation on a display device. The techniques,methods, systems, and other mechanisms described herein includeprocesses for altering information associated with one or more frames ofa first compressed video file to allow for accurate rendering of frameswithin the first compressed video file prior to a transition to a secondcompressed video file. In some implementations, time stamp informationfor one or more frames of a compressed video file is altered. Forexample, a time stamp of a particular frame of a compressed video fileis altered prior to transmission of the particular frame by a videotransmission system such that the altered time stamp indicates a timethat has already passed. The particular frame can be decoded by adisplay system that receives the particular frame so that informationincluded in the particular frame can be used in rendering ofsubsequently received frames. After the subsequent frame that depends onthe particular frame for accurate rendering has been transmitted, thevideo transmission system can begin transmitting frames for a secondcompressed video file. The altered time stamp for the particular frameindicating a time in the past indicates to the display system that theparticular frame has already been presented and should not be presentedagain even if the particular frame had not actually been previouslypresented by the display system. The display system therefore bypassesdisplay of the particular frame, but is able to use information includedin the particular frame to render the subsequently received frame. Afterpresentation of the subsequently received frame, the display systembegins to display frames from the second compressed video file accordingto presentation time stamps for the frames of the second compressedvideo files.

In general, one innovative aspect of the subject matter described inthis specification can be embodied in a computing device having a memorystoring data and instructions and one or more processors that executeinstructions stored on the memory. The instructions can cause the one ormore processors to execute instructions that perform actions includingreceiving, by a computing system, first compressed video content;receiving, by the computing system, second compressed video content;identifying, by the computing system, a splice point for the firstcompressed video content; identifying a particular frame in the firstcompressed video content that precedes the splice point; determiningthat the particular frame depends on information included in asubsequent frame of the first compressed video content that is after thesplice point; altering, by the computing system and in response todetermining that the particular frame depends on information included inthe subsequent frame, time stamp information of the subsequent frame;and transmitting, by the computing system and to a video presentationsystem, the particular frame, the subsequent frame along with thealtered time stamp information, and at least a portion of the secondcompressed video content.

These and other embodiments can each optionally include one or more ofthe following features. Altering the time stamp information of thesubsequent frame can include reading a presentation time stamp valueassociated with the subsequent frame; subtracting a particular valuefrom the presentation time stamp value; storing the resulting value ofsubtracting the particular value from the presentation time stamp valueas a new presentation time stamp for the subsequent frame. Theparticular value can be between 5 ms and 150 m. The particular value canbe approximately 20 ms. The particular value can be determined based onone or more characteristics of a frame buffer of the video presentationsystem. The one or more characteristics of the frame buffer can includean amount of time that the video presentation system retains decodedframes in the frame buffer. Transmitting the particular frame, thesubsequent frame along with the altered time stamp information, and atleast a portion of the second compressed video content comprisestransmitting the subsequent frame prior to the particular frame andtransmitting the at least a portion of the second compressed videocontent after the particular frame.

The computing system can identify a second splice point for the secondcompressed video content. The computing system can identify anadditional frame in the second compressed video content that is afterthe second splice point. The computing system can determine that theadditional frame depends on information included in a preceding frame ofthe second compressed video content that is before the second splicepoint. The computing system can alter time stamp information of thepreceding frame. The computing system can alter the time stamp inresponse to determining that the additional frame depends on informationincluded in the preceding frame, and prior to transmitting theparticular frame, the subsequent frame along with the altered time stampinformation, and at least a portion of the second compressed videocontent. The at least the portion of the second compressed video caninclude the additional frame and the preceding frame.

Particular embodiments of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. A video presentation can be created from two ormore compressed video files without requiring the individual video filesto be decompressed and recompressed. All of the information needed todecode and display a frame of a compressed video file that relies ininformation in a subsequent frame and that is to be displayed prior to asplice point with another video can be provided to a display systemwithout requiring the subsequent frame to be displayed. A frame thatrelies on information from a subsequent frame to be decoded anddisplayed can be transmitted to, decoded by, and displayed by a videodisplay system without requiring the frame to be decoded and re-encodedas a different type of frame. A compressed video transmission system cantransmit portions of compressed video files that are spliced together tocreate a single video presentation without having to convert B-framesfrom the first compressed video that are to be presented just prior tothe splice point to I-frames or P-frames. Prevents loss of contentnecessary for rendering one or more frames prior to a splice point in acombined video presentation that rely on information included in frameslocated after the splice point in an original video file on which thecombined video presentation is based. Avoids presentation of frameslocated after a splice point in an original video file on which acombined video presentation is based while ensuring that the informationincluded in those frames is available for decoding of other frames.

Other features, aspects, and advantages of the subject matter willbecome apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example system for splicing compressedvideo files for presentation as a single video presentation.

FIG. 2A illustrates an example frame display sequence for a primarycontent compressed video file.

FIG. 2B illustrates an example frame display sequence for a secondarycontent compressed video file.

FIG. 2C illustrates an example frame transmission sequence for acombined video made up of frames from two different compressed videofiles.

FIG. 3 is a flow diagram of an example process for splicing compressedvideo files.

FIG. 4 is a schematic diagram of an example of a computer system thatmay be used to implement the systems and methods described in thisdocument.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Video compression allows for video content to be stored and transmittedmore efficiently while using less resources. For example, transmittingvideo in a compressed format frees up more data transmission resourcesthan transmission of non-compressed video, thereby freeing up resourcesfor transmission of additional video content or transmission to a largernumber of recipient devices. Examples of video compression formatsinclude standards set by the Moving Picture Experts Group (MPEG) oftenreferred to as “MPEG” formats. Examples of MPEG formats used in avariety of applications include the MPEG-2 formats and the MPEG-4formats.

In some compressed video formats (such as, for example, the H.264/MPEG-4AVC (MPEG-4 Part 10) format) some frames of a compressed video areentirely or partly interceded, i.e., the frames are rendered usinginformation derived from other frames in the compressed video asreference. In some video formats, frames can be classified according toif they are inter-coded or intra-coded (non-inter-coded) frames. Anintra-coded frame or Intra-frame (often abbreviated as “I-frame”) can bedecoded independently of any other frames in a compressed video. Inother words, all of the information necessary for rendering an I-frameis contained within the I-frame itself and rendering of the I-frame doesnot require information from other frames.

Furthermore, inter-coded frames (frames that require information fromone or more other frames to be decoded) can be divided into twoclassifications: Predicted-frames (often abbreviated as “P-frames”) andBidirectional-frames (often abbreviated as “B-frames”). P-frames canimprove compression by exploiting the temporal (over time) redundancy ina video. A P-frame stores only the difference in image from the frame(either an I-frame or P-frame) immediately preceding it (this referenceframe is sometimes referred to as an “anchor frame”). A display systemreceiving a P-frame uses information included in the anchor frame alongwith the difference in image information included in the P-frame itselfto render the P-frame.

B-frames are similar to P-frames in that they are also inter-codedframes and can improve compression by exploiting temporal redundancy ina video. However, B-frames rely on information included in both a priorframe and subsequent frame for decoding (e.g., the B-frame relies on twoanchor frames). The B-frame relies on both information included in theprior frame and the subsequent frame along with difference informationincluded in the B-frame itself. A display system rendering a B-framemust therefore decode the next I-frame or P-frame that sequentiallyfollows the B-frame in the video before the B-frame can be decoded anddisplayed. In some implementations, the B-frame relies on onlyinformation from a subsequent frame in the compressed video and does notrely on information from a prior frame in the compressed video.

In some video compression formats, frames have one or more associatedtime stamps. For example, a frame in a compressed video can have anassociated presentation time stamp (“PTS”) and/or an associated decodingtime stamp (“DTS”). In some implementations, the PTS and DTS are offsetvalues from a reference starting time (such as the beginning of avideo). The PTS can identify the order in which frames are to bepresented by a video display system by identifying times at which theframes are to be displayed. PTS values can also be used to determinewhen data can be discarded from a buffer storing received frames. TheDTSs for frames in a compressed video identify the order in which framesare to be decoded by identifying times at which the frames are to bedecoded.

In video compression formats that utilize B-frames, the DTS and PTS forsome frames may differ. That is the order in which frames of acompressed video are to be decompressed differs from the order in whichthe frames of the compressed video are to be displayed. This is becausea B-frame relies on information included in a frame that is to bedisplayed after the B-frame. For example, suppose that frames 4 and 7 ofa compressed video are I-frames while frame 5 of the compressed video isa B-frame. Frame 5 may rely on information included in both frames 4 and7 to be decoded and displayed. Thus, the video display system displayingthe compressed video must decode frame 7 prior to decoding frame 5 eventhough frame 7 is to be displayed after frame 5. In this example, frame7 would include a DTS that is earlier than the DTS for frame 5 so thatframe 7 is decoded prior to frame 5. Decoding frame 7 prior to frame 5ensures that the information included in frame 7 is available for use indecoding and rendering frame 5. In this example, frame 7 would have aPTS that is later than the PTS for frame 5 (and frame 6) because frame 7is to be presented after frame 5.

In some implementations, frames in a compressed video may be associatedwith a PTS but not with a DTS. For example, in some video compressionformats, the frames are decoded in the order in which they are receivedand are presented in the order determined by the PTS of each frame. Forexample, a video stream might include frames 1, 2, 3, 4, and 5 that areto be presented in numerical order. Each of frames 1, 2, 3, 4, and 5have an associated PTS that collectively identify that frames 1, 2, 3,4, and 5 are to be displayed in in the order: 1, 2, 3, 4, 5. In thisexample, assume frame 3 is a B-frame that relies on frame 4 for at leastsome information. A video transmitting system that transmits the framesof the compressed video can transmit the frames in the order: 1, 2, 4,3, 5. The video display system decodes the frames in the order receivedand therefore frame 4 is decoded prior to frame 3 which makes theinformation included in frame 4 available for use in decoding andrendering frame 3. However, the frames are presented in the orderidentified by the respective PTSs of the frames. In this example, theframes are stored in a frame buffer after they are decoded. Therefore,frame 4 can be decoded prior to frame 3 and the decoded frame 4 isstored in the frame buffer. The decoded frame 4 is therefore availablefor use in decoding frame 3, which is then stored in the frame buffer inits decoded format. The video display system can then retrieve frames 3and frame 4 from the frame buffer for presentation in the order dictatedby the respective PTSs of the frames.

Turning to FIG. 1, a block diagram of an example system 100 for splicingvideo content is depicted. In general, a data processing system such asa content server 102 can receive compressed video content from varioussources and provide different segments of video content to video displaysystems for presentation as a single video presentation to users of thevideo display systems. In the example shown, the content server 102receives primary video content 104 from a primary content provider 106and secondary video content 108 from a secondary video content provider110.

In the illustrated example, the content server 102 receives one or moreprimary video content segments 104 that are provided by a primarycontent provider 106. In some implementations, the primary video content104 can be live or pre-recorded television programs, streaming videocontent, movies, or any other appropriate video media content. In thisexample, the primary video content 104 is compressed video contentstored in a compressed video format such as, for example, MPEG-2(sometimes referred to as H.262), MPEG-2.2 (i.e. MPEG-2 Part 2, alsoknown as ISO/IEC 13818-2), MPEG-4, MPEG4 part 10, ITU-T H.264 (sometimesreferred to as Advanced Video Codec (AVC), ISO/IEC 14496-10, MPEG-4 Part10 Advanced Video Coding, or Joint Video Team (NT)). In someimplementations, the primary video content 104 includes a series ofI-frames, P-frames, and B-frames that can be decoded by a display systemto display the primary video content 104.

The content server 102 receives one or more secondary video contentsegments 108 from the secondary video content provider 110. Thesecondary video content 108 can also be stored in a compressed videoformat such as those discussed above with respect to the primary videocontent 104. In some implementations, the primary video content 104represents content that is intended for a wider audience of users whilethe secondary video content 108 is content that is intended for asmaller subset of users. For example, the primary video content 104 canbe a television program intended for a national audience while thesecondary video content 108 can be content that is distributed (e.g.,transmitted) specifically to a particular geographic region or locality(e.g., a country, state, or city). For example, the primary videocontent 104 can be a national news program while the secondary videocontent 108 is a regional news segment or weather forecast. As anotherexample, the primary content 104 can be a program about national sportsleagues while the secondary video content 108 contains information onlocal sports teams (e.g., local high school sports). As yet anotherexample, the primary video content 104 can be a comedy program while thesecondary video content 108 is a promotion for a local business that isspecific to a geographic region where the business is located. In eachof these examples, the secondary video content 108 that is distributedto different geographic regions at any specific time can includedifferent presentation content.

In some implementations, the primary video content 104 is longer formcontent while the secondary video content 108 is shorter form content.For example, the primary video content 104 can be an hour long programwhile the secondary video content 108 can be a series of shorter oneminute videos. In some implementations, the primary video content 104and the secondary video content 108 are the same relative type ofcontent. For example, both the primary video content 104 and thesecondary video content 108 may be ten minute documentaries that arepresented one after the other.

The content server 102 splices together the primary video content 104with the secondary video content 108 to generate a combined videopresentation 112. For example, the content server 102 can splice thesecondary video content 108 onto the end of the primary video content104 such that after the last frame of the primary video content 104 isdisplayed, the first frame of the secondary video content 108 isdisplayed. In some implementations, rather than splicing the secondaryvideo content 108 onto the end of the primary video content 104, thecontent server 102 splices some or all of the secondary video content108 with a portion of the primary video content 104 at a splice pointidentified within the primary video content 104. The content server 102can splice the primary video content 104 with the secondary videocontent 108 such that the combined video presentation 112 begins byshowing a portion of the primary video content 104 and switches topresenting all or a portion of the secondary video content 108 at adesignated splice point. The portion of the secondary video content 108that is included in the combined video presentation 112 can start at thebeginning of the secondary video content 108, or can start at a splicepoint identified within the secondary video content 108. Numeroustechniques for identifying splice points within the primary videocontent 104 and the secondary video content 108 are known in the art,and therefore not discussed in this document.

The combined video presentation 112 is then transmitted to displaysystems for presentation to viewers through distribution channels suchas a series of one or more communications networks. For example, thecontent server 102 can provide transmit the combined video presentation112 to a television system 114 that includes, for example, a highdefinition television 116 and a set-top box 118 in communication withthe television 116. The set-top box 118 can, for example, receive thecombined video presentation 112 from the content server 102 through acable television service and decode the frames of the combined videopresentation 112 for display on the television 116. In someimplementations, the television system 114 includes the television 116but does not require the set-top box 118.

As additional examples, the content server 102 can provide the combinedvideo presentation 112 for presentation to a user through a mobiledevice 120 (e.g., a smartphone) receiving streaming content from acellular network provider, or through a computer 122 receiving contentfrom a streaming media website (e.g., through an Internet, WAN, or LANconnection). In some implementations, other devices may be used toreceive the combined video presentation 112, such as, for example, PDAs,tablet devices, or other devices capable of displaying video content.

The content server 102 employs processes for splicing the primary andsecondary video content 104 and 108 such that all information necessaryto display the frames of the primary video content 104 prior to thesplice point and the information necessary to display the frames of thesecondary video content 108 after the splice point is provided to thedisplay system (e.g., the television system 114, mobile device 120, orcomputer 122). For example, turning to FIG. 2A, an example frame displaysequence 200 for primary video content 104 is displayed. The primarycontent frame display sequence 200 shows the order in which a series offrames of the primary video content 104 are to be displayed insituations in which the primary video content 104 is not spliced withother video content. In the example shown, the display sequence forframes X1-X9 is displayed. FIG. 2B shows an example frame displaysequence 210 for secondary video content 108. The secondary contentframe display sequence 210 shows the order in which a series of framesof the secondary video content 108 are to be displayed in situations inwhich the secondary video content 108 is not spliced with other videocontent. In the example shown, the display sequence for frames Y1-Y4 isdisplayed.

Returning to FIG. 2A, during a splicing process for splicing the primaryvideo content 104 with the secondary video content 108, the contentserver 102 identifies a splice point 202 in the primary video content104. For example, the splice point 202 can be identified by meta dataincluded with the primary video content 104, or can be identified byanalyzing some or all of the frames of the primary video content 104.The content server 102 can then identify any B-frames in the primaryvideo content 104 that occur within a specified number of frames priorto the splice point 202. For example, the content server 102 can accessinformation for the 20 frames in the primary video content 104 thatimmediately proceed the splice point 202 and determine if any of those20 frames are B-frames. In the example shown in FIG. 2A, assume that thecontent server 102 has identified that frames X2 and X4 are bothB-frames. In this example, the content server 102 does not analyzeframes X5-X9 to determine if they are B-frames because they are locatedafter the splice point 202 in the primary content frame display sequence200.

The content server 102 then determines if any of the identified B-frameslocated prior to the splice point 202 rely on information in a framethat is located after the splice point 202 in the primary content framedisplay sequence 200. As described above, a B-frame in a compressedvideo file relies on information included in a frame that is locatedafter the B-frame in a display sequence. Therefore, in this example inwhich frames X2 and X4 have been identified as B-frames, each of X2 andX4 rely on information contained in a frame that appears later in theprimary content frame display sequence 200 to be properly decoded anddisplayed by a video display system. Continuing with this example, thecontent server 102 can identify that frame X2 relies on informationincluded in frame X3 to be decoded and that frame X4 relies oninformation contained in frame X6 to be decoded. For example, frame X2may include a pointer that identifies frame X3 as a frame on which frameX2 depends to be properly rendered and frame X4 may include a pointerthat identifies frame X6 as a frame on which frame X4 depends to beproperly rendered. With regard to frame X2's dependence on frame X3, thecontent server 102 determines that frame X3 is located prior to thesplice point 202 in the primary content frame display sequence 200. Thismeans that frame X3 is to be included in the combined video presentation112 resulting from the splicing of primary video content 104 andsecondary video content 108 because all frames prior to the splice point202 in the primary content frame display sequence 200 are to bedisplayed as part of the combined video presentation 112. Being as frameX3 is to be displayed as part of the combined video presentation 112,the content server 102 does not alter any information associated withframe X3.

With regard to frame X4's dependence on information included in frameX6, the content server 102 determines that frame X6 is located after thesplice point 202 in the primary content frame display sequence 200. Thisindicates that frame X6 is not to be displayed in the combined videopresentation 112 when the primary video content 104 is spliced with thesecondary video content 108. Based on the determinations that frame X4depends on information from frame X6 and that frame X6 is located afterthe splice point 202 in the primary content frame display sequence 200,the content server 102 can modify time stamp information for frame X6 toallow frame X6 to be provided to a video display system (for use inproperly decoding frame X4) while also ensuring that frame X6 will notbe displayed by the video display system. For example, the contentserver 102 can alter a presentation time stamp (PTS) for frame X6 bychanging the PTS to a time that is prior to the presentation time offrame X4. For example, the content server 102 can alter the PTS forframe X6 such that the altered PTS for frame X6 is the same as the PTSfor frame X2. In some implementations, the process can be successfullyimplemented by changing the PTS for frame X6 to any PTS that is equal toor earlier than the PTS for frame X3 (i.e., the frame that immediatelyproceeds frame X4). In implementations in which the frame X6 has anassociated decoding time stamp (DTS), the content server 102 does notalter the DTS of the frame X6.

The content server 102 can then generate the combined video presentation112 by taking frames from the primary video content 104 and frames fromthe secondary video content 108 and placing them in a frame transmissionsequence 220 shown in FIG. 2C. The combined video frame transmissionsequence 220 shows the order in which the content server 102 transmitsframes for the combined video presentation 112 to a video display system(such as the television system 114, for example). The combined videoframe transmission sequence 220 includes all frames from the primarycontent frame display sequence 200 that occur prior to the splice point202, plus frame X6 due to frame X4's dependency on information includedin frame X6. The combined video frame transmission sequence 220 alsoincludes the frames from secondary content frame display sequence 210.

The combined video frame transmission sequence 220 includes the frameX6, but the frame X6 will not be presented by the video display systembecause the content server 102 has altered the PTS for frame X6 toindicate a time that is equal to or prior to the PTS for frame X3. Inthe example shown in FIG. 2C, the video display system receives theframes in the order shown by the combined video frame transmissionsequence 220. The video display system will decode the frames in theorder they are received and store the decoded frames in a frame buffer(sometimes referred to as a decoded picture buffer or “DPB”). In thisexample, frame X3 is received and decoded prior to frame X2 becauseframe X2 depends on frame X3 to be properly decoded and displayed.Similarly, frame X6 is received and decoded prior to frame X4 becauseframe X4 depends on frame X6 to be properly decoded and displayed.Because frame X6 is decoded and stored in the frame buffer prior toframe X4, the information included in frame X6 necessary for decodingframe X4 is available at the time that the video display system decodesframe X4. Upon decoding frame X4, decoded frame X4 is stored in theframe buffer.

The video display system then displays the frames in the order specifiedby the PTSs of the frames. In this example, the video display systemretrieves frame X1 from the frame buffer and displays frame X1 firstfollowed by frames X2, X3, and X4. The video display system can analyzethe altered PTS for frame X6 and determine that frame X6 has alreadybeen presented (even though in reality, frame X6 was never presented)due to frame X6 having an altered PTS that indicates a time equal to orprior to the PTS for frame X3. Frame X6 is therefore not presented bythe video display system. The video display system moves on to displayframes Y1-Y2 (i.e., frames from the secondary video content 108) afterdisplaying frame X4. Frame Y1 can be, for example, an I-frame andtherefore can be decoded and displayed by the video display systemwithout use of information from another frame. The subsequent frames(frames Y2-4) can be I-frames or P-frames, for example.

In some implementations, the altered PTS for frame X6 is determinedusing one or more factors relating to the PTSs of other frames, thelocation of frame X6 in the primary content frame display sequence 200,the location of frame X4 in the primary content frame display sequence200, the size of the frame buffer, or attributes associated with theframe buffer of the video display system. For example, in someimplementations, the altered PTS for frame X6 is selected to be equal toor prior to the PTS for the frame which immediately proceeds the framethat depends on frame X6. In the example in FIG. 2A, frame X4 dependsfrom frame X6. Therefore, the PTS for frame X6 is altered to be equal toor prior to the PTS for frame X3 (the frame immediately prior to frameX4 in the primary content frame display sequence 200). In someimplementations, the PTS for frame X6 is determined, in part, based onthe amount of time frames are kept in the frame buffer. For example, thePTS for frame X6 must be selected such that frame X6 is still in theframe buffer when frame X4 is decoded. Therefore, the altered PTS forframe X6 must not be so far back in time that the video display systemwould purge frame X6 from the frame buffer prior to frame X4 beingdecoded. For example, if the frame buffer retains frames for 100milliseconds (ms) prior to purging frames from the frame buffer, thecontent server 102 should set the altered PTS for frame X6 such that itis not more than 100 ms prior to the PTS (or alternatively, the DTS) forframe X4. In other words, the altered PTS for frame X6 can be set suchthat the altered PTS does not exceed a threshold time prior to a timestamp associated with the frame that depends on frame 6. The thresholdtime can be determined based on characteristics of the frame buffer,such as an amount of time that frames are kept in the frame buffer.

In some implementations, the PTS for frame X6 is altered by a setamount. For example, the PTS for frame X6 is altered by 20 ms. In otherwords, the 20 ms is subtracted from the original PTS for frame X6 tocreate the altered PTS for frame X6. As another example, the contentserver 102 generates the altered PTS by subtracting 100 ms from theoriginal PTS for frame X6. In other examples, values of 10 ms, 15 ms, 25ms, or 30 ms can be subtracted from the original PTS for frame X6 tocreate the altered PTS for frame X6. In some implementations, a timefrom between 5 ms-150 ms is subtracted from the original PTS for frameX6 to create the altered PTS for frame X6. In some implementations, thetime subtracted from the original PTS for frame X6 is in the range of 10ms-120 ms, 15 ms-100 ms, or 20 ms-50 ms.

In some implementations, a threshold value for frame presentation by avideo display device is used to determine the adjustment factor for thealtered PTS for frame X6. For example, a decoder in a video displaydevice may release video frames when they match the decoder system timeclock (STC) within a certain threshold value. The altered PTS for frameX6 can be determined using the threshold value for displaying frames.

In some implementations, when generating the combined video presentation112, the content server 102 adjusts the time stamp (PTS) for each framein the secondary video content 108 by a delta such that the PTSs for theframes of the secondary video content 108 dictate that the frames of thesecondary video content 108 are presented after the frames of theprimary video content 104 included in the combined video presentation112. In the example shown in FIG. 2C, the PTSs for each of frames Y1-Y4can be altered by a delta value such that frames Y1-Y4 are presentedafter frame X4 in the combined video presentation 112. The delta can be,for example, based off of the PTS of frame X4. In some implementations,the delta can be the PTS for frame X4 plus an amount of time that isrequired between presentations of each frame. For example, if the amountof time between each frame is 5 ms, the delta added to the PTSs forframes Y1-Y4 can be the PTS for frame X4 plus 5 ms. In someimplementations, the PTS for the first non-presented frame of theprimary video content 104 is used as the delta. For example, in theexample shown in FIGS. 2A-C, the delta added to the PTSs of frames Y1-Y4can be the PTS of frame X5 from the primary content frame displaysequence 200.

In some implementations, rather than being decoded according to theorder they are received, the video display system decodes framesaccording to decoding time stamps (DTSs) associated with the frames. Insuch implementations, the content server 102 can alter the PTS for aframe while leaving the original DTS for the frame to ensure that theframe is decoded at the proper time while also ensuring that the framewill not be presented by the video display system. For example, thecontent server 102 can alter the PTS for frame X6 (e.g., by subtracting20 ms from the original PTS for frame X6) so that the video displaysystem does not present frame X6. The content server 102 can leave theDTS for frame X6 as is so that frame X6 is decoded prior to frame X4 andthe information included in frame X6 is available for use in decodingframe X4.

In some alternate embodiments, methods other than or in addition toaltering a time stamp for a frame can be used to signal to the videodisplay system that the frame should not be displayed. For example,refer to the previous example in which frame X4 depends on informationfrom frame X6 to be properly decoded, but frame X6 is after the splicepoint 202 in the primary content frame display sequence 200 (and shouldtherefore not be displayed as part of the combined video presentation112). In some implementations, the content server 102 can set a “do notdisplay” flag associated with frame X6 prior to transmitting the frameX6 to the video display system. The video display system can identifythat the “do not display” flag associated with the frame X6 is set andbypass display of frame X6. As another example, the PTS for the frame X6can be set to a special “flag” value to indicate that the frame X6 is tobe decoded and stored in the frame buffer by the video display system,but is not to be displayed as part of the combined video presentation112. For example, the content server 102 can set the PTS for frame X6 to“−1” or “0.” This special PTS value for frame X6 can signal to the videodisplay system that frame X6 should not be displayed.

In some implementations, a portion of the secondary video content 108may be spliced with a portion of the primary video content 104 atlocation other than the beginning of the primary video content 104. Forexample, the content server 102 can identify a splice point in thesecondary video content 108. The splice point can indicate a point (inthe time dimension) at which presentation of frames from the secondaryvideo content 108 within the combined video presentation 112 shouldbegin. For example, referring to FIG. 2B, the splice point can belocated between frames Y2 and Y3, this can indicate that only frames Y3and Y4 (and subsequent frames) should be displayed as part of thecombined video presentation 112 and frames Y1 and Y2 should not bedisplayed in the combined video presentation 112. Continuing with thisexample, Y3 may be a P-frame that depends on information from frame Y2to be properly rendered. Thus, the video display system that receivesand displays the combined video presentation 112 will require frame Y2to accurately decode frame Y3. The content server 102 can alter at timestamp, such as the PTS, for frame Y2 in ways similar to those describedabove with respect to frame X6 such that frame Y2 can be transmitted tothe video display system without being displayed by the video displaysystem. For example, the content server 102 can subtract 20 ms from thePTS for frame Y2. Other factors or methods for generating the alteredPTS for frame Y2, such as those described above for frame X6, can beutilized. The content server 102 can then transmit frames Y2 and Y3 tothe video display system as part of the combined video presentation 112.The video display system will decode frame Y2 and store it in a framebuffer so that the information in frame Y2 is later available for use indecoding frame Y3. However, due to the altered time stamp for frame Y2,frame Y2 will not be displayed by the video display system. This isbecause when the video display system reads the PTS for frame Y2, thevideo display system will determine that frame Y2 has already beendisplayed (even though in reality frame Y2 has not been displayed) andwill therefore bypass display of frame Y2.

FIG. 3 is a flow diagram of an example process 300 for splicingcompressed video content. In some implementations, the process 300 maybe performed by the content server 102 of FIG. 1. The process 300 beginsby receiving primary content and secondary content (310). The primaryand secondary content can be, for example, compressed video content. Theprimary compressed video content can be, for example, a televisionprogram while the secondary compressed video content can be, forexample, a local news segment or a promotional video. The primary andsecondary video content can be received from the same content provideror from different content providers.

A splice point in the primary content is identified (320). For example,meta data included with primary video content can identify a splicepoint for the primary video content. As another example, a computingsystem (such as the content server 102) that receives the primary videocontent can analyze the primary video content to identify a splicepoint. The computing system can analyze features of frames in theprimary video content to identify one or more ideal splice points. Insome implementations, the splice point is identified by a cue toneincluded with the primary video content identifies a splice point in theprimary video content.

A particular frame in the primary content that is located (in the timedimension) prior to the splice point and depends on a subsequent framelocated (in the time dimension) after the splice point is identified(330). For example, presentation time stamps (PTSs) associated withframes in the primary video content can dictate a presentation order forthe frames in the primary video content. The computing system canidentify a frame in the primary video content that is prior to thesplice point that requires information included in a subsequent frame tobe properly rendered and decoded. The computing system can thendetermine that the subsequent frame is after the splice point (andtherefore should not be presented as part of a combined videopresentation that includes frames from the primary video content priorto the splice point). The particular frame can be, for example, aB-frame that relies on information included in a subsequent frame forproper decoding and presentation by a video display system.

A time stamp for the subsequent frame is altered (340). For example, aPTS for the subsequent frame can be altered by subtracting an amount oftime from the original PTS for the subsequent frame. For example, 20 mscan be subtracted from the PTS for the subsequent frame. As anotherexample, 100 ms can be subtracted from the PTS for the subsequent frame.In some implementations, various factors are used in creating an alteredPTS for a frame. For example, the amount of time that a frame is kept inthe buffer of a video display system can be used as a factor in creatingthe altered PTS for the subsequent frame. As another example, a lengthof a “presentation-zone” time for a video display system can be used asa factor in altering the PTS for the subsequent frame. The“presentation-zone” can be, for example, a threshold time period arounda current system time clock time in which frames having PTSs within thattime period are displayed. The length of this presentation time periodcan be used in determining the altered PTS for the subsequent frame. Thetime stamp for the subsequent frame is altered such that the alteredtime stamp will be interpreted by a video display system that receivesthe subsequent frame in a way that causes the video display system todecode but not display the subsequent frame. For example, upon readingthe altered time stamp for the subsequent frame, the video displaysystem can make a determination that the frame has already beendisplayed (even though in reality, the subsequent frame has not beendisplayed by the video display system). The video display system willsubsequently not display the subsequent frame.

The subsequent frame (including the altered time stamp information), theparticular frame, and at least a portion of the secondary content aretransmitted to a video display system (350). For example, referring toFIG. 2C, the combined video frame transmission sequence 220 includesframe X6 (the subsequent frame which includes an altered time stamp) andframe X4 (the particular frame that depends on the subsequent frame tobe properly decoded and displayed) from the primary video content 104and frames Y1-Y4 from the secondary video content 108. In someimplementations, the frames are transmitted in an order in which theyare to be decoded. In some implementations, prior to transmitting theportion of the secondary content, time stamps associated with each ofthe frames of the secondary content are altered by a delta factor suchthat the transmitted frames of the secondary content are displayed by avideo display system after the particular frame is displayed.

FIG. 4 is a schematic diagram of an example of a computer system 400that may be used to implement the systems and methods described in thisdocument. The system 400 can be used for the operations described inassociation with the method 300 according to one implementation.

The system 400 includes a processor 410, a memory 420, a storage device430, and an input/output device 440. Each of the components 410, 420,430, and 440 are interconnected using a system bus 450. The processor410 is capable of processing instructions for execution within thesystem 400. In one implementation, the processor 410 is asingle-threaded processor. In another implementation, the processor 410is a multi-threaded processor. The processor 410 is capable ofprocessing instructions stored in the memory 420 or on the storagedevice 430 to display graphical information for a user interface on theinput/output device 440.

The memory 420 stores information within the system 400. In oneimplementation, the memory 420 is a computer-readable medium. In oneimplementation, the memory 420 is a volatile memory unit. In anotherimplementation, the memory 420 is a non-volatile memory unit.

The storage device 430 is capable of providing mass storage for thesystem 400. In one implementation, the storage device 430 is acomputer-readable medium. In various different implementations, thestorage device 430 may be a floppy disk device, a hard disk device, anoptical disk device, or a tape device.

The input/output device 440 provides input/output operations for thesystem 400. In one implementation, the input/output device 440 includesa keyboard and/or pointing device. In another implementation, theinput/output device 440 includes a display unit for displaying graphicaluser interfaces.

Embodiments of the subject matter and the operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them. Embodiments of the subject matterdescribed in this specification can be implemented as one or morecomputer programs, i.e., one or more modules of computer programinstructions, encoded on computer storage medium for execution by, or tocontrol the operation of, data processing apparatus. A computer storagemedium can be, or be included in, a computer-readable storage device, acomputer-readable storage substrate, a random or serial access memoryarray or device, or a combination of one or more of them. The computerstorage medium can also be, or be included in, one or more separatephysical components or media (e.g., multiple CDs, disks, or otherstorage devices).

The operations described in this specification can be implemented asoperations performed by a data processing apparatus on data stored onone or more computer-readable storage devices or received from othersources.

The term “data processing apparatus” encompasses all kinds of apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, a system on a chip, or multipleones, or combinations, of the foregoing The apparatus can includespecial purpose logic circuitry, e.g., an FPGA (field programmable gatearray) or an ASIC (application-specific integrated circuit). Theapparatus can also include, in addition to hardware, code that createsan execution environment for the computer program in question, e.g.,code that constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, a cross-platform runtimeenvironment, a virtual machine, or a combination of one or more of them.The apparatus and execution environment can realize various differentcomputing model infrastructures, such as web services, distributedcomputing and grid computing infrastructures.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, declarative orprocedural languages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, object, orother unit suitable for use in a computing environment. A computerprogram may, but need not, correspond to a file in a file system. Aprogram can be stored in a portion of a file that holds other programsor data (e.g., one or more scripts stored in a markup languagedocument), in a single file dedicated to the program in question, or inmultiple coordinated files (e.g., files that store one or more modules,sub-programs, or portions of code). A computer program can be deployedto be executed on one computer or on multiple computers that are locatedat one site or distributed across multiple sites and interconnected by acommunication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform actions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. The essential elements of a computer area processor for performing actions in accordance with instructions andone or more memory devices for storing instructions and data. Generally,a computer will also include, or be operatively coupled to receive datafrom or transfer data to, or both, one or more mass storage devices forstoring data, e.g., magnetic, magneto-optical disks, or optical disks.However, a computer need not have such devices. Moreover, a computer canbe embedded in another device, e.g., a mobile telephone, a personaldigital assistant (PDA), a mobile audio or video player, a game console,a Global Positioning System (GPS) receiver, or a portable storage device(e.g., a universal serial bus (USB) flash drive), to name just a few.Devices suitable for storing computer program instructions and datainclude all forms of non-volatile memory, media and memory devices,including by way of example semiconductor memory devices, e.g., EPROM,EEPROM, and flash memory devices; magnetic disks, e.g., internal harddisks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROMdisks. The processor and the memory can be supplemented by, orincorporated in, special purpose logic circuitry.

To provide for interaction with a user, embodiments of the subjectmatter described in this specification can be implemented on a computerhaving a display device, e.g., a CRT (cathode ray tube) or LCD (liquidcrystal display) monitor, for displaying information to the user and akeyboard and a pointing device, e.g., a mouse or a trackball, by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide for interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback, e.g.,visual feedback, auditory feedback, or tactile feedback; and input fromthe user can be received in any form, including acoustic, speech, ortactile input. In addition, a computer can interact with a user bysending documents to and receiving documents from a device that is usedby the user; for example, by sending web pages to a web browser on auser's client device in response to requests received from the webbrowser.

Embodiments of the subject matter described in this specification can beimplemented in a computing system that includes a back-end component,e.g., as a data server, or that includes a middleware component, e.g.,an application server, or that includes a front-end component, e.g., aclient computer having a graphical user interface or a Web browserthrough which a user can interact with an implementation of the subjectmatter described in this specification, or any combination of one ormore such back-end, middleware, or front-end components. The componentsof the system can be interconnected by any form or medium of digitaldata communication, e.g., a communication network. Examples ofcommunication networks include a local area network (“LAN”) and a widearea network (“WAN”), an inter-network (e.g., the Internet), andpeer-to-peer networks (e.g., ad hoc peer-to-peer networks).

The computing system can include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other. In someembodiments, a server transmits data (e.g., an HTML page) to a clientdevice (e.g., for purposes of displaying data to and receiving userinput from a user interacting with the client device). Data generated atthe client device (e.g., a result of the user interaction) can bereceived from the client device at the server.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments of the subject matter have been described.Other embodiments are within the scope of the following claims. In somecases, the actions recited in the claims can be performed in a differentorder and still achieve desirable results. In addition, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In certain implementations, multitasking and parallelprocessing may be advantageous.

What is claimed is:
 1. A computer implemented method comprising:receiving, by a computing system, first compressed video content;receiving, by the computing system, second compressed video content;identifying, by the computing system, a splice point for the firstcompressed video content; identifying a particular frame in the firstcompressed video content that precedes the splice point; determiningthat the particular frame depends on information included in asubsequent frame of the first compressed video content that is after thesplice point; altering, by the computing system and in response todetermining that the particular frame depends on information included inthe subsequent frame, time stamp information of the subsequent frame,wherein altering the time stamp information of the subsequent framecomprises: reading a presentation time stamp value associated with thesubsequent frame; subtracting a particular value from the presentationtime stamp value; and storing the resulting value of subtracting theparticular value from the presentation time stamp value as a newpresentation time stamp for the subsequent frame; and transmitting, bythe computing system and to a video presentation system, the particularframe, the subsequent frame along with the altered time stampinformation, and at least a portion of the second compressed videocontent; wherein the particular value is between 5 ms and 150 ms.
 2. Themethod of claim 1, wherein the particular value is approximately 20 ms.3. The method of claim 1, wherein the particular value is determinedbased on one or more characteristics of a frame buffer of the videopresentation system.
 4. The method of claim 3, wherein the one or morecharacteristics of the frame buffer comprises an amount of time that thevideo presentation system retains decoded frames in the frame buffer. 5.The method of claim 1, wherein transmitting the particular frame, thesubsequent frame along with the altered time stamp information, and atleast a portion of the second compressed video content comprisestransmitting the subsequent frame prior to the particular frame andtransmitting the at least a portion of the second compressed videocontent after the particular frame.
 6. A computer implemented methodcomprising: receiving, by a computing system, first compressed videocontent; receiving, by the computing system, second compressed videocontent; identifying, by the computing system, a first splice point forthe first compressed video content; identifying a particular frame inthe first compressed video content that precedes the first splice point;determining that the particular frame depends on information included ina subsequent frame of the first compressed video content that is afterthe first splice point; altering, by the computing system and inresponse to determining that the particular frame depends on informationincluded in the subsequent frame, time stamp information of thesubsequent frame; transmitting, by the computing system and to a videopresentation system, the particular frame, the subsequent frame alongwith the altered time stamp information, and at least a portion of thesecond compressed video content; identifying, by the computing system, asecond splice point for the second compressed video content; identifyingan additional frame in the second compressed video content that is afterthe second splice point; determining that the additional frame dependson information included in a preceding frame of the second compressedvideo content that is before the second splice point; and altering timestamp information of the preceding frame, wherein altering the timestamp information of the preceding frame is performed (1) by thecomputing system, (2) in response to determining that the additionalframe depends on information included in the preceding frame, and (3)prior to transmitting the particular frame, the subsequent frame alongwith the altered time stamp information, and at least a portion of thesecond compressed video content.
 7. The method of claim 6, whereinaltering the time stamp information of the subsequent frame comprises:reading a presentation time stamp value associated with the subsequentframe; subtracting a particular value from the presentation time stampvalue; and storing the resulting value of subtracting the particularvalue from the presentation time stamp value as a new presentation timestamp for the subsequent frame.
 8. The method of claim 6, wherein the atleast the portion of the second compressed video includes the additionalframe and the preceding frame.
 9. A tangible, non-transitory recordablemedium having recorded thereon instructions, that when executed, cause acomputing system to perform actions that comprise: receiving firstcompressed video content; receiving second compressed video content;identifying a splice point for the first compressed video content;identifying a particular frame in the first compressed video contentthat precedes the splice point; determining that the particular framedepends on information included in a subsequent frame of the firstcompressed video content that is after the splice point; altering, inresponse to determining that the particular frame depends on informationincluded in the subsequent frame, time stamp information of thesubsequent frame, wherein altering the time stamp information of thesubsequent frame comprises: reading a presentation time stamp valueassociated with the subsequent frame; subtracting a particular valuefrom the presentation time stamp value; and storing the resulting valueof subtracting the particular value from the presentation time stampvalue as a new presentation time stamp for the subsequent frame; andtransmitting, to a video presentation system, the particular frame, thesubsequent frame along with the altered time stamp information, and atleast a portion of the second compressed video content; wherein theparticular value is between 5 ms and 150 ms.
 10. The recordable mediumof claim 9, wherein the particular value is approximately 20 ms.
 11. Therecordable medium of claim 9, wherein the particular value is determinedbased on one or more characteristics of a frame buffer of the videopresentation system.
 12. The recordable medium of claim 11, wherein theone or more characteristics of the frame buffer comprises an amount oftime that the video presentation system retains decoded frames in theframe buffer.
 13. The recordable medium of claim 9, wherein transmittingthe particular frame, the subsequent frame along with the altered timestamp information, and at least a portion of the second compressed videocontent comprises transmitting the subsequent frame prior to theparticular frame and transmitting the at least a portion of the secondcompressed video content after the particular frame.
 14. A tangible,non-transitory recordable medium having recorded thereon instructions,that when executed, cause a computing system to perform actions thatcomprise: receiving first compressed video content; receiving secondcompressed video content; identifying a first splice point for the firstcompressed video content; identifying a particular frame in the firstcompressed video content that precedes the first splice point;determining that the particular frame depends on information included ina subsequent frame of the first compressed video content that is afterthe first splice point; altering, in response to determining that theparticular frame depends on information included in the subsequentframe, time stamp information of the subsequent frame; transmitting, toa video presentation system, the particular frame, the subsequent framealong with the altered time stamp information, and at least a portion ofthe second compressed video content; identifying a second splice pointfor the second compressed video content; identifying an additional framein the second compressed video content that is after the second splicepoint; determining that the additional frame depends on informationincluded in a preceding frame of the second compressed video contentthat is before the second splice point; and altering time stampinformation of the preceding frame, wherein altering the time stampinformation of the preceding frame is performed (1) by the computingsystem, (2) in response to determining that the additional frame dependson information included in the preceding frame, and (3) prior totransmitting the particular frame, the subsequent frame along with thealtered time stamp information, and at least a portion of the secondcompressed video content.
 15. The recordable medium of claim 14, whereinaltering the time stamp information of the subsequent frame comprises:reading a presentation time stamp value associated with the subsequentframe; subtracting a particular value from the presentation time stampvalue; and storing the resulting value of subtracting the particularvalue from the presentation time stamp value as a new presentation timestamp for the subsequent frame.
 16. The recordable medium of claim 14,wherein the at least the portion of the second compressed video includesthe additional frame and the preceding frame.
 17. A computing systemcomprising: a communications interface; one or more computer readablememories storing executable instructions; and one or more processorsthat interact with the one or more computer readable memories andexecute instructions that cause the computing system to performoperations comprising: receiving first compressed video content;receiving second compressed video content; identifying a splice pointfor the first compressed video content; identifying a particular framein the first compressed video content that precedes the splice point;determining that the particular frame depends on information included ina subsequent frame of the first compressed video content that is afterthe splice point; altering, in response to determining that theparticular frame depends on information included in the subsequentframe, time stamp information of the subsequent frame, wherein alteringthe time stamp information of the subsequent frame comprises: reading apresentation time stamp value associated with the subsequent frame;subtracting a particular value from the presentation time stamp value;and storing the resulting value of subtracting the particular value fromthe presentation time stamp value as a new presentation time stamp forthe subsequent frame; and transmitting, to a video presentation system,the particular frame, the subsequent frame along with the altered timestamp information, and at least a portion of the second compressed videocontent; wherein the particular value is between 5 ms and 150 ms. 18.The computing system of claim 17, wherein the operations furthercomprise: identifying a second splice point for the second compressedvideo content; identifying an additional frame in the second compressedvideo content that is after the second splice point; determining thatthe additional frame depends on information included in a precedingframe of the second compressed video content that is before the secondsplice point; and altering time stamp information of the precedingframe, wherein altering the time stamp information of the precedingframe is performed (1) by the computing system, (2) in response todetermining that the additional frame depends on information included inthe preceding frame, and (3) prior to transmitting the particular frame,the subsequent frame along with the altered time stamp information, andat least a portion of the second compressed video content.
 19. Thecomputing system of claim 17, wherein the particular value isapproximately 20 ms.
 20. The computing system of claim 17, wherein theparticular value is determined based at least in part on an amount oftime that the video presentation system retains decoded frames in theframe buffer.